HARRISBURG >> Drought and warm nights are conspiring to mute the annual uproar of yellows, reds and purples in Pennsylvania's woods.

The peak season for most fall foliage in the state is three weeks away, but leaves, green leaves, are falling in the forest. The autumn chill also may be delayed this year, according to forecasters.

Crisp nights would fire up spectacular displays. A little rain would help keep the leaves on the trees.

AccuWeather Meteorologist Anthony Sagliani said a very warm pattern will remain in place over the Great Lakes, Appalachians and Northeast.

"Through the first half of October, we expect a generally warm pattern," he said. "There are a couple of signs for a cool shot or two, but any long-lived cold snaps appear unlikely."

"A warm period during fall may lower the intensity of autumn colors," said Edward Dix, forest program specialist with the Pennsylvania Department of Conservation and Natural Resources.

Think yellow and brown, without much red.

"A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays," Dix said. "During these days, lots of sugars are produced in the leaf, but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions — lots of sugar and light — spur production of the brilliant anthocyanin pigments, which tint reds, purples, and crimson."

Yellow and gold colors remain fairly constant from year to year because carotenoids are always present in leaves, he said.

The leaves stop producing green chlorophyll in the fall, and the carotenoids shine through.

Shortening days and lengthening nights trigger trees to prepare for winter, so the time for leaf peeping is around the same time every year. The best time for viewing fall foliage in southcentral Pennsylvania is Oct. 20 to 31.

"A season without early cold nights may be slower to progress through the change," Dix said. "Rather than a 'peak' with all trees changing color within a week to 10 days, we observe a 'plateau' of gradual change over about two to three weeks in any location."

Already the small leaves on blackgum trees are turning deep red in Michaux State Forest. They are among the earliest to change color. Dogwood and maple follow with oaks among the last to turn.

"Early turning species like dogwood and blackgum seem to be least effected by variations in weather," Dix said.

Western Franklin County has been abnormally dry in the past month or so. Groundwater conditions are abnormally dry, according to water data from the U.S. Geological Survey monitoring well near Chambersburg.

"In dry locations trees may show the stress by dropping leaves early," Dix said. "This is a response to drought stress that can happen anytime and is not effected by the season. If stressed trees drop their leaves early they will not be participating in the seasonal show."

Most trees however are not stressed as easily as field crops, he said. Humid air passing through the forest canopy on a cool morning provides relief as water condenses onto the leaves and falls to the ground keeping forest soils moist.

"Most of our forest canopy trees are 80 to 100 years old," Dix said. "They are survivors. The weaklings died in droughts over the last century."

The impact of temperature on the quality of colors is subjective and varies from tree to tree and location to location, he said.

"A stormy cold front can bring an abrupt end to the fall foliage season," Dix said. Cells die where the leaf stalk attaches to the twig. When the cells break down the leaf falls. Wind and rain can shorten the show.